In process-measurement and automation technology, physical parameters of a fluid flowing in a pipe, such as mass flow rate, density, and/or viscosity, are frequently measured by means of meters which, using a vibratory transducer traversed by the fluid and a measuring and control circuit connected thereto, induce reaction forces, such as Coriolis forces corresponding to the mass flow rate, inertial forces corresponding to the density, or friction forces corresponding to the viscosity, in the fluid in a non-invasive manner, and derives therefrom a measurement signal representing the respective mass flow rate, viscosity, and/or density of the fluid.
Such vibratory transducers are disclosed, for example, in WO-A 01/33174, WO-A 00/57141, WO-A 98/07009, WO-A 95/16897, WO-A 88/03261, U.S. Pat. No. 6,006,609, U.S. Pat. No. 5,796,011, U.S. Pat. No. 5,301,557, U.S. Pat. No. 4,876,898, U.S. Pat. No. 4,524,610, EP-A 553 939, or EP-A 1 001 254.
To conduct the fluid, each of the transducers comprises at least one flow tube held in a support frame and having a bent or straight tube segment which in operation, driven by an electromechanical excitation assembly, is caused to vibrate in order to produce the above-mentioned reaction forces. To sense vibrations of the tube segment, particularly inlet-side and outlet-side vibrations, the transducers each comprise a sensor arrangement which responds to motions of the tube segment.
Aside from such vibration transducers, electromagnetic transducers or transducers evaluating the transit time of ultrasonic waves transmitted in the direction of fluid flow, particularly transducers based on the Doppler principle, are frequently used in process-measurement and automation technology for in-line measurements. Since the basic construction and the operation of such electromagnetic transducers are sufficiently described in EP-A 1 039 269, U.S. Pat. No. 6,031,740, U.S. Pat. No. 5,540,103, U.S. Pat. No. 5,351,554, or U.S. Pat. No. 4,563,904, for example, and the basic construction and the operation of such ultrasonic transducers are sufficiently described in U.S. Pat. No. 6,397,683, U.S. Pat. No. 6,330,831, U.S. Pat. No. 6,293,156, U.S. Pat. No. 6,189,389, U.S. Pat. No. 5,531,124, U.S. Pat. No. 5,463,905, U.S. Pat. No. 5,131,279, or U.S. Pat. No. 4,787,252, for example, a detailed explanation of these principles of measurement can be dispensed with at this point.
For clarification it should be mentioned that within the scope of the invention, “non-invasive transducers” means those transducers which do not comprise any flow bodies which are immersed in the fluid and serve to influence its flow for the purpose of producing measurement effects. By contrast, within the scope of this invention, those transducers which, in order to measure the fluid, produce vortices in the fluid flow or use baffles, bluff bodies, floats, or orifice plates are regarded as “invasive transducers”. Such invasive transducers are also familiar to those skilled in the art and are sufficiently described, for example, in WO-A 01/20282, WO-A 97/22855, U.S. Pat. No. 6,352,000, U.S. Pat. No. 6,003,384, U.S. Pat. No. 5,939,643, U.S. Pat. No. 5,922,970, U.S. Pat. No. 5,458,005, U.S. Pat. No. 4,716,770, U.S. Pat. No. 4,476,728, U.S. Pat. No. 4,445,388, U.S. Pat. No. 4,437,350, U.S. Pat. No. 4,339,957, EP-A 690 292, EP-A 684,458, DE-A 39 04 224, DE-A 38 10 889, DE-A 17 98 360, or DE-A 100 01 165.
During the use of non-invasive in-line transducers it turned out that in the case of inhomogeneous fluids, particularly of multiphase fluids, the measurement signals produced, in spite of the viscosity and density being maintained virtually constant, particularly under laboratory conditions, are subject to considerable nonreproducible variations and may thus become practically unusable for the measurement of the respective physical parameter.
In U.S. Pat. No. 4,524,610, a possible cause of this problem in the operation of vibratory transducers is indicated, namely the fact that parasitic inhomogeneities introduced by the fluid into the flow tube, such as gas bubbles, may be trapped at the inside wall of the tube. To avoid this problem, it is proposed to install the transducer so that the straight flow tube is in an essentially vertical position, so that the trapping of such parasitic, particularly gaseous, inhomogeneities is prevented.
This, however, is a very specific solution which is only conditionally realizable, particularly in industrial process measurement technology. On the one hand, the pipe into which the transducer is to be inserted would have to be adapted to the transducer and not vice versa, which probably cannot be conveyed to the user. On the other hand, the flow tubes, as mentioned, may also have a curved shape, so that the problem cannot be solved by adapting the mounting position, either. It also turned out that the above-described distortions of the measurement signal cannot be appreciably reduced even if a vertically installed straight flow tube is used. Variations in the measurement signal in the presence of a flowing fluid cannot be prevented in this manner, either.